1. Field of the invention
The present invention relates to a toner used for image forming processes such as electrophotography and electrostatic recording, and more particularly to a magnetic toner.
2. Related Background Art
A large number of methods have been conventionally known as electrophotography, as disclosed in U.S. Pat. No. 2,297,691, Japanese Patent Publication No. 42-23910 (U.S. Pat. No. 3,666,363) and Japanese Patent Publication No. 43-24748 (U.S. Pat. No. 4,071,361), etc. In general, copies are obtained by forming an electrostatic latent image on a photosensitive member utilizing a photoconductive material and according to various means, subsequently developing the latent image by the use of toner to form it into a visible image, and transferring the toner image to a transfer medium such as paper if necessary, followed by fixing by the action of heat, pressure or heat-and-pressure.
Developing methods in which an electrostatic latent image is formed into a visible image by the use of a toner are also known in variety. For example, developing methods such as the magnetic brush development as disclosed in U.S. Pat. No. 2,874,063, the cascade development as disclosed in U.S. Pat. No. 2,618,552, the powder cloud development as disclosed in U.S. Pat. No. 2,221,776, the fur brush development and the liquid development are known in the art. Of these developing methods, the magnetic brush development, the cascade development and the liquid development that employ a developer mainly composed of a toner and a carrier have been widely put into practical use. These methods are superior methods that can obtain a good image in a relatively stable state, but on the other hand have the problems that the carrier may undergo deterioration and the mixing ratio between the toner and the carrier may change.
To avoid such problems, various proposals have been made on developing methods in which a one-component type developer comprised of only a toner is used. In particular, many advantages are seen in methods in which a developer comprised of toner particles having magnetic properties is used.
U.S. Pat. No. 3,909,258 proposes a method in which development is carried out using a magnetic toner having an electrical conductivity. According to this method, a conductive magnetic toner is supported on a cylindrical conducting sleeve having a means for generating an magnetic field, as exemplified by a magnet, and then the toner is brought into contact with an electrostatic image to carry out development. In this instance, in a developing zone, toner particles form a conducting path between the surface of a recording member and the surface of the sleeve. Charges are led from the sleeve to the toner particles through this conducting path, and the toner particles are adhered to an image area by the Coulomb force that acts between the toner particles and the image area of the electrostatic image. Thus, toner particles are adhered to the image area and the electrostatic image is developed. The developing method using this conductive magnetic toner is a superior method that has avoided the problems involved in the conventional two-component developing methods. On the other hand, since the toner is conductive, this method has the problem that it is difficult to electrostatically transfer a developed image from the recording member to an image support member such as plain paper.
As a developing method that uses a high-resistivity magnetic toner feasible for electrostatic transfer of a developed image, there is a method in which the dielectric polarization of toner particles is utilized. Such a method, however, has the problems that it can only achieve a fundamentally low developing speed and the developed image can not have a sufficient density.
As another developing method that uses a high-resistivity magnetic toner, a method is known in which toner particles are triboelectrically charged by the friction between the toner particles or each particle and a sleeve, and the toner particles thus charged are brought into contact with an electrostatic image supporting member to carry out development. This method, however, has the problems that the triboelectricity of a toner tends to become insufficient because of less contact occasions between toner particles and a friction member such as the sleeve, and the charged toner particles tend to be agglomerated on the sleeve as a result of an increase in the Coulomb force between the toner particles and the sleeve.
Japanese Patent Application Laid-open No. 55-18656 (corresponding to U.S. Pat. No. 4,395,436) proposes a novel developing method that has eliminated the above problems. According to this method, a magnetic toner is coated on a sleeve in a very small thickness, and the magnetic toner is triboelectrically charged. Then the magnetic toner layer is brought into close proximity to an electrostatic image to carry out development. This method can obtain an excellent toner image for the following reasons: i) The coating of a magnetic toner on a sleeve in a very small thickness has increased the occasions of contact between the sleeve and the toner and thus has enabled sufficient triboelectric charging, ii) the magnetic toner is supported on the sleeve by the action of a magnetic force, and also a magnet built in the sleeve and the magnetic toner are relatively moved so that the agglomeration between magnetic toner particles can be loosened and the magnetic toner can be brought into sufficient friction with the sleeve, and iii) since the magnetic toner is supported on the sleeve by the action of a magnetic force and the development is carried out in such a state that the magnetic toner on the sleeve is opposed to the electrostatic image without contact with it, the background fog can be prevented. It is characteristic of a developing device used in such a developing method that its structure can be simple and its size can be made very small.
For example, in a high-speed machine that employs such a developing device, a photosensitive member (e.g., a photosensitive drum) can have room around its circumference. Hence, there are advantages such that it becomes easy to arrange several developing devices having toners with different colors so that colors of toners can be changed at a single operation, or to use laser light simultaneously with analogue light so that the page number or letters can be written simultaneously with copying.
Since in a small-sized machine, the developing device can be made light-weight and small, and hence the technique for the above developing device is nowadays indispensable for making adaptation of copying machines for personal use.
In the field of printers, as typified by a small-sized laser beam printer (LBP), it has become very important for a developing device to require only a small space and be simple and light-weight, in order to provide noiseless and a high-speed printout compared with dot printers or thermal printers.
On the other hand, since this developing system is characterized by the simple, light-weight and small-sized developing device, there is the problem that toners used in this system must have higher performance than conventional toners (e.g., toners used in two-component developers) so that the image quality, durability and stability which are excellent as a whole can be obtained. The performance of the system often reflects the performance of such magnetic toners as it is.
In particular, as for copying machines themselves, those in which digital latent images are used have become available in place of conventional ones of an analogue type, and hence it has become possible to form latent images with much more resolution than ever. A magnetic toner that can well follow such fine latent images must be capable of performing development in a high resolution. In addition, since the copying machines are also tending toward high-speed copying, a magnetic toner used therefor must now be able to satisfy the high resolution, high-speed development and high durability.
In the case of printers also, there are similar demands for high performance. From the viewpoint of high durability, printers, which are used as output units of computers, must print out very frequently and are required to have much severer performance for durability than copying machines.
Toner images are no longer satisfactory only if they are monotonously black. In the case of copying machines, it is particularly demanded that photographs can also be faithfully reproduced, in other words, their middle tones or halftones can be reproduced. In the copying machine of a digital latent image type, the halftone is expressed according to the difference in the density of lines, and hence it becomes difficult to reproduce halftones of an original unless the lines have the same thickness.
Such tone reproduction is highly required particularly in the printers of a digital latent image type. Conventional magnetic toners have a tendency not to be well capable of forming images stably having the same halftones at the both initial stage and final stage of the duration for printing on a large number of sheets.
As for environmental stability, copying machines and LBPs are nowadays often used in severer environments. Because as copying machines have been adapted to personal use or LBPs have become inexpensive, they have become wide-spread for domestic use. In particular, under a condition that a copying machine is placed in a poor environment at home for many days, and during which several copies are occasionally taken, magnetic toners are required to have a very high performance in view of image stability and environmental stability.
Japanese Patent Application Laid-open No. 56-91242 (corresponding to U.S. Pat. No. 4,485,613) and Japanese Patent Publication No. 59-27901 (corresponding to U.S. Pat. No. 4,495,268) propose magnetic toners in which a magnetic material of cubic crystals preferably having a particle diameter of not less than 0.3 .mu.m is used. Such a magnetic material is noted to have a good development efficiency and transfer efficiency and cause less black spots around line images and less background stain. However, when this magnetic material is used in a magnetic toner having a volume average particle diameter of smaller than 10 .mu.m, the amount of static charge of the magnetic toner tends to increase. Therefore, taking account of environmental stability, it is sometimes difficult to control the amount of static charge of the magnetic toner. It is proposed in Japanese Patent Application Laid-open No. 1-112253 (corresponding to European Patent Application Publication No. 0314459) to make smaller the particle diameter of a magnetic toner. According to this proposal, the volume average particle diameter of a magnetic toner which usually distributes between 10 and 14 .mu.m is controlled to be 4 to 9 .mu.m and also the particle size distribution of the magnetic toner is defined. Use of this magnetic toner brings about improvements in fine-line reproduction, halftone reproduction and tone reproduction.
However, in order to answer the recent severe demands, it is sought to further improve the magnetic toners having small particle diameters.
Among materials used in magnetic toners, the magnetic material, in particular, has a great influence on the performance of a magnetic toner since it is contained in an amount of from 20 to 70% by weight based on the total magnetic toner.
Japanese Patent Application Laid-open No. 58-169153 proposes a magnetic toner containing magnetic powder having such a particle size distribution that 50% number average particle diameter ranges from 0.3 to 1.0 .mu.m, a 50% weight average particle diameter ranges from 0.4 to 1.3 .mu.m, and the particle diameter giving a maximum value in the number particle size distribution ranges from 0.4 to 1.3 .mu.m. This magnetic toner is noted to give good image fidelity and stability, well remove background fogging development, and also promise a high resolution and a high density, as well as good environmental characteristics.
It is true that such a magnetic toner has performance sufficient for practical use when used in conventional analogue type machines, but this toner can no longer be said to be sufficient for the high-speed development and high durability when used in recently available high-speed machines of not less than 50 sheets per minute, and also for the high gradation, the high resolution to digital latent images and the fine-line reproduction in digital latent images.
In particular, in order to stably form images with halftones for a long period of time, the above magnetic toner can not be said to have sufficient performance, and the above magnetic powder is insufficient when used in a magnetic toner with a small particle diameter.
Japanese Patent Application Laid-open No. 58-87951 (corresponding to U.S. Pat. No. 4,543,312 proposes that magnetic materials having such particle size distributions that the 50% particle diameter (calculated based on volume) ranges from 1.5 to 4.5 .mu.m, the 20% particle diameter (calculated based on volume) ranges from 1.0 to 4.0 .mu.m and the 75% particle diameter (calculated based on volume) ranges from 2.5 to 6.0 .mu.m should be used. These magnetic powders are for use in color magnetic toners, and are not suitable for use in black image formation. These magnetic toners are insufficient in blackness, and are not preferable.
Japanese Patent Publication No. 62-51208 proposes a magnetic toner that posesses improved dispersibility of a magnetic material to a resin by using a spherical magnetic material and can thereby form a toner image with a high image density. It is true that the spherical magnetic material has such advantages, but it tends to have higher electrical resistance. Circumstances may become severer when the magnetic toner containing the spherical magnetic material has a small particle diameter, which is liable to be charged up in a high-speed machine or small-sized machine. In general, when a magnetic toner has been charged up, it becomes difficult for the magnetic toner to be separated from a toner-carrying member such as a developing sleeve, so that the image density may sometimes be lowered. Moreover, the fogging phenomenon that the background is stained may sometimes occur.
For example, in order to simply achieve a high resolution and a high fine-line reproduction by the use of such a magnetic toner having a small particle diameter, it might be understood to decrease the amount of the magnetic toner applied to fine line images so that no excessive magnetic toner may form black spots around line images. This method, however, brings about a lowering of image density at black solid areas, and hence is not preferable. An attempt to simply increase the image density tends to cause stains on the background. In particular, if the magnetic toner is left to stand in an environment of low temperature and low humidity for a long period of time, the stains on the background may sometimes become conspicuous. When magnetic toners are used, it is not easy to achieve a high image density and a high resolution and to make an image free from stains on the background.